Synroc Wasteform

Radioactive Waste Management Appendix 1

Synroc is a suite of technologies which provide the most effective and durable means of immobilising various forms of intermediate- and high-level radioactive wastes for disposal.

Synroc is basically a ceramic made from several natural minerals which together incorporate into their crystal structures nearly all of the elements present in high level radioactive waste.

Recent developments are of specialised forms to immobilise plutonium, and of composite glass-ceramic wasteforms.

Synroc is a particular kind of "Synthetic Rock", invented in 1978 by the late Professor Ted Ringwood of the Australian National University. It has since diversified, but generally speaking is an advanced ceramic comprising geochemically stable natural titanate minerals which have immobilised uranium and thorium for billions of years. These can incorporate into their crystal structures nearly all of the elements present in high-level radioactive waste (HLW) and so immobilise them. Originally some 57% of Synroc was titanium dioxide (rutile, TiO2).

For over 30 years, the Australian Science & Technology Organisation (ANSTO) has been developing the technology. Today, Synroc can take various forms depending on its specific use and can be tailored to immobilise particular components in the intermediate-level wastes (ILW) or HLW. The original form, Synroc-C*, was intended mainly for the immobilisation of liquid HLW arising from the reprocessing of light water reactor fuel. However, by 1980 those reprocessing used fuel had chosen borosilicate glass as the medium for immobilisation because it was the most technically mature technology then.

* The main minerals in Synroc-C are hollandite (BaAl2Ti6O16), zirconolite (CaZrTi2O7) and perovskite (CaTiO3). Zirconolite and perovskite are the major hosts for long-lived actinides such as plutonium (Pu), though perovskite is principally for strontium (Sr) and barium (Ba). Hollandite principally immobilises caesium (Cs), along with potassium (K), rubidium (Rb) and barium. Synroc-C can hold up to 30% HLW by weight.

Since the 1980s, different forms of Synroc have been developed to deal with wastes for which there is no current disposition route, particularly military radioactive wastes, including a substantial amount of plutonium. Other applications have been developed related to the partitioning and transmutation of wastes. This involves partitioning HLW into separate components, some of which can then be transmuted, or changed, into different forms which are less radioactive or shorter-lived (usually by neutron bombardment in a reactor or accelerator). Those which are not suitable for transmutation can then be immobilised in Synroc, since the hot isostatic pressing (HIP) involved copes with volatile radionuclides (such as Tc & Cs) and eliminates off-gases in a one-step process. ANSTO has developed the HIP technology to increase waste loadings.

The waste form is the key component of the immobilization process, as it determines both waste loading (concentration), which directly impacts cost (due to volume reduction), as well as the chemical durability, which determines environmental risk (especially in relation to highly mobile and long-lived I, Tc & Cs). Synroc is also thermally robust and can incorporate high concentrations of caesium and strontium, with high radiogenic heat output. Other advantages claimed relative to vitrification are greater processing flexibility with elimination of off-gas emissions. To achieve maximum cost savings and optimum performance the Synroc waste forms are tailored to suit the particular characteristics of nuclear waste to be immobilised rather than adopting a single one-size fits all approach.

Background

Early research and development on Synroc and its properties was carried out at the Australian Nuclear Science and Technology Organisation (ANSTO) Research Laboratories at Lucas Heights, NSW, and at the Australian National University (ANU) in Canberra. From the early 1980s funding was provided by the Australian Government. A pilot plant to manufacture Synroc using only non-radioactive material was designed and constructed at Lucas Heights. Synroc became the flagship of an ANSTO program which has now broadened into other wasteforms and maintains an international profile as SynrocANSTO.

At the Australian Government's request, a Synroc Study Group (SSG) was set up in 1989 by four Australian companies, BHP (now BHP Billiton), CRA (now Rio Tinto), Energy Resources of Australia (ERA) and Western Mining Corporation (now part of BHP Billiton), in association with ANSTO and ANU to consider commercialisation. The situation was complicated by the fact that existing spent fuel reprocessing plants in France and the UK, and the plant now built in Japan, are committed to the use of borosilicate glass for immobilisation.

Although the advantages of the Synroc approach have been well recognised internationally, billions of dollars have been invested in glass technology for high level waste (HLW) arising from the reprocessing of nuclear fuel. Whilst glass is appropriate for a large fraction of HLW, considerable quantities of waste exist that are very difficult to incorporate in glass and it is these waste streams in particular that ANSTO is targeting, with its tailored range of waste forms. Initially, ANSTO oriented its Synroc research to military wastes, and in particular to the clean-up problems faced by the US Department of Energy (DoE) at their Savannah River site and at Hanford.

Development for US military wastes

Early alternatives to Synroc-C were developed for military wastes at Savannah River. Because of problems involving high levels of non-radioactive contaminants in the wastes, the new formulation, Synroc-D, contained nepheline - (Na, K)AlSiO4 instead of hollandite as host for Cs, Rb and Ba. Another variant, Synroc-F, was rich in pyrochlores - (Ca, Gd, U, Pu, Hf)2Ti2O7, and was developed for the disposal of unreprocessed spent fuel from light water and Candu reactors.

In 1994, ANSTO began working with the US Lawrence Livermore National Laboratory (LLNL) to develop a Synroc variant for plutonium disposition. Early efforts focused on zirconolite-rich titanates, since zirconolite is the most durable of the original Synroc phases and hence a natural host for actinides in general and plutonium in particular. These were also loaded with neutron absorbers such as gadolinium, hafnium and samarium to reduce the risk of criticality during processing and later. Zirconolite variants containing hollandite and rutile were produced either by sintering the ingredients in air at 1375°C or by hot isostatic pressing of calcines at 1280°C and 150 MPa in argon.

But this work was eventually dropped in favour of the pyrochlore-rich ceramic. This was found to be more efficient for immobilising the uranium contained in the waste stream without the loss of rare-earth neutron absorbers from the crystalline structure (which tended to be displaced by U). The plutonium feedstocks have significant levels of diverse impurities.

The specialised form of Synroc which has emerged for this plutonium program is a pyrochlore-rich titanate ceramic with an increased loading of natural uranium and neutron absorbers (Gd, Sm, Hf) needed for nuclear criticality control. Pyrochlore is similar to zirconolite and can incorporate up to 50% by mass of PuO2 and/or UO2. There is twice as much U as Pu (23% : 11.5% by mass) so that high U-238 levels will ensure additional criticality control as Pu-239 decays to U-235. There is as much Hf and Gd (each) as Pu. The baseline wasteform is 95% pyrochlore and 5% rutile (TiO2) with Hf replacing one tenth of the Ti. However, there are large variations according to feedstock composition and impurities.

In 1997 Synroc was tested with real HLW using technology developed jointly by ANSTO and the US DoE's Argonne National Laboratory. The project to compress the waste in a remotely controlled hot cell provided the first remote demonstration of hot isostatic pressing on a commercial scale. HIP is good for retaining volatile radionuclides such as Tc, Cs, Sr & I, and gives rise to no off-gas emissions

In 1998 DoE chose the pyrochlore-rich form of Synroc from among 70 contending treatments for HLW management. This process adapts some of the ceramic technology from mixed-oxide fuel production. The DoE hoped to have the plutonium immobilisation facility operational at Savannah River by 2007, with the product going to geological disposal. Cans containing pressed and sintered ceramic discs containing plutonium would be surrounded in canisters by vitrified HLW to provide an external gamma radiation barrier which would deter attempts at plutonium recovery.

In anticipation, ANSTO set up a joint venture with Cogema of France through their US subsidiaries to bid for the contract to build the plutonium immobilisation plant. The venture also included US company Burns & Roe, and Battelle. The bid was submitted in mid 2000 but in April 2001 the DoE announced that it was deferring immobilisation plans in favour of the MOX plant, representing the other policy track for plutonium disposition. The MOX could be used in five TVA reactors at Browns Ferry and Sequoyah. In 2010 a Federal Register notice confirmed that the can-in-a-canister plutonium disposal option had been dropped.

ANSTO continued discussing with DoE the use of Synroc for immobilising a range of problematic HLW primarily arising from Cold War nuclear activities. It is relevant to note the advantages of the pyrochlore-rich ceramic over the lanthanide borosilicate glass which was the alternative: it is more chemically insoluble, giving better proliferation resistance; it is easy and safer to process; it is criticality-safe since it incorporates neutron absorbers in the same phase as the actinides and also depleted uranium; higher actinide waste loadings are possible resulting in about half the volume of solid waste; and neutron dose rate to workers is very much less.

Since then Synroc was one of four technologies evaluated by DOE to treat 4400 cubic metres of granulated HLW calcine at Idaho. For this, ASNSTO developed glass-ceramic wasteforms. Early in 2010 the DOE chose the Synroc with hot isostatic pressing (HIP) for the Idaho calcines. This had the best combination of volume reduction, chemical durability, and modular plant construction giving future versatility. In addition, using HIP technology significantly reduces radioactive volatility during high-temperature processing compared with competing process technologies. DOE estimated it would save some $5 billion in disposal costs at the eventual geological repository compared with vitrification technology, due to having some 50% less volume. ANSTO in fact developed a special glass-ceramic waste form for this contract (see below).

Composite wasteforms

Glass-ceramic composites which combine the process and chemical flexibility of glass with the superior chemical durability of ceramics have been developed and have achieved very high waste loadings (50-80%). They can be tailored for the particular wastes and applied to those which are intractable due to their complex and heterogeneous chemistry. Major cost and time savings result.

Where there is a lot of either sodium or silicon in the wastes, these can form a glass as part of a durable glass-ceramic composite wasteform. ANSTO has developed Synroc-glass combinations where the radioactive materials are typically incorporated into extremely durable crystalline titanate phases such as zirconolite and pyrochlore (which will hold actinides, particularly plutonium), within a glass matrix. Waste loadings of up to 80% by weight have been demonstrated in such composite wasteforms formed from dry sintered wastes. These composite wasteforms are typically melted at 1200-1400°C, although the use of hot isostatic pressing (1150ºC and 14 MPa) has some advantages.

The composite process was originally developed to deal with wastes at the DoE Hanford site which are highly contaminated with sodium nitrate and nitrites. The wastes were being vitrified in borosilicate glass (with about 20% waste loading), but space is at a premium and a synroc-glass composite would reduce volumes. Calcined waste (with no Pu) from the 1960s at DoE's Idaho laboratory (INEEL) were also part of the program, and in 2010 the Synroc composite was chosen to immobilise these.

Composite wasteforms are also the subject of a collaborative research program with the French Atomic Energy Commission (CEA). This includes developing Synroc-glass waste forms using French cold-crucible melting technology. These achieve a 50% waste loading and may be used for French partitioning and transmutation programs.

In 2005 ANSTO entered an agreement with Nexia Solutions, now part of the UK's new National Nuclear Laboratory (NNL), to use a composite Synroc glass-ceramic waste form for 5 tonnes of impure plutonium waste which had been stored for 50 years at Sellafield in UK and could not be immobilised in glass. The glass-ceramic mix is subject to hot isostatic pressing. ANSTO and NNL have built and operated a demonstration plant there. The process will result in substantial cost savings to the UK government and can be applied to other actinide - notably plutonium - waste streams.

Synroc for ILW from Mo-99 production

In connection with its molybdenum-99 production facility (from LEU) at Lucas Heights near Sydney, ANSTO is building a new Synroc plant to immobilise the ILW liquid wastes. This SyMo plant will reduce waste volume by 90% after two years decay before HIP. It will produce 320 cans per year, each 30 litres (1.4 tonnes). The plant will treat legacy acid wastes as well as alkaline ones from ongoing Mo-99 production. The final storage volume with HIP Synroc will be 1% of the same wastes cemented for disposal. ANSTO hopes that the Synroc technology “will become the benchmark for waste treatment in the production of Mo-99 radiopharmaceuticals."

Waste partitioning and conditioning

Radioactive waste partitioning is usually considered in the context of partitioning and transmutation (P/T) strategies aimed at reducing the long-term potential hazard associated with HLW destined for geological disposal. P/T involves the separation of minor actinides and long-lived fission products in advanced reprocessing and their transmutation into products of greatly reduced half-lives. Because of practical difficulties in achieving high separation factors which are required for efficient transmutation, P/T concepts have increased interest in conditioning certain partitioned waste streams in durable matrices such as Synroc.

ANSTO has demonstrated that significant reductions in waste volume for the partitioning/conditioning strategy can be achieved through immobilisation in Synroc-C of the heat-generating radionuclides, i.e., Cs-137, Sr-90, and Cm-244 together with long-lived Tc-99, while the remaining waste is immobilised in glass. Extended near-surface storage of the Synroc would be required for 100 to 200 years, while disposal of borosilicate glass containing the remaining radionuclides could be carried out immediately. The combination of partitioned caesium and strontium with curium-244 ensures that the effects of alpha-decay processes on the waste form durability are minimised through self-annealing of alpha-decay damage due to the radiogenic decay heat. The presence of curium, which is not likely to be recycled for transmutation, will become more important in the future with reprocessing of mixed oxide (MOX) fuels.

A zirconolite-rich Synroc can also be considered for the conditioning of the Am/Cm/rare-earth stream from HLW partitioning. The very low release rates from Synroc of Am, Cm and their long-lived daughters suggest that there may be little incentive on cost and radiological benefit grounds for the transmutation of Am and Cm. Separation of Am-241, which is a major source of Np-237, is essential in P/T strategies aimed at significantly decreasing the long-term radiological risk from geological disposal. Efficient separation from each other of these minor actinides and the rare earths on an industrial scale will require significant technological improvements and could involve additional costs to limit radiological exposures to operators of subsequent fuel fabrication plants.

Appendix:

Long-term commercial prospects as perceived by Synroc Study Group

In 1991 the SSG published a Progress Report, identifying five options for the commercialisation:

licensing of the Synroc patents and "know-how" overseas;

participation by Australian companies in overseas plant(s) using Synroc;

establishment in Australia of an international toll reprocessing plant for spent fuel, with immobilisation of the separated HLW in Synroc and its return to the customers along with recycled fuel (uranium and some plutonium);

establishment overseas of an integrated spent fuel management industry. This would take spent fuel from customers, provide transport, temporary storage, reprocessing and immobilisation of HLW in Synroc, with the return of recycled fuel to customers, and final geological disposal of the immobilised HLW;

establishment in Australia of an integrated spent fuel management industry with both Australian and international participation, with final disposal of HLW immobilised in Synroc on an Australian territorial site.

The SSG found the potential economic benefits to Australia much greater for the fifth option, although the political obstacles were recognised to be substantial. The concept of long-term storage and/or final disposal in a country which is neither the original user of the fuel nor its reprocessor was not new; - it was proposed by China in the early 1980s. This option is now being explored further on a non-commercial basis by Pangea's successor, ARIUS, and by Russia.

The International Atomic Energy Agency (IAEA), which is concerned with worldwide safety and security standards for the nuclear energy industry, has promoted the concept of regional fuel cycle facilities for many years.